1. Field of the Invention
The present invention relates to a transformer, especially to a transformer with an externally-mounted rectifying circuit board.
2. Description of the Prior Art
With the development of IT industry, electronic device is required to be compact, multi-functional, user-friendly, etc. To manufacture a miniature electronic device, electronic elements and circuit boards applied in the electronic device should be as small as possible. For most electronic devices, the size of the electronic device mainly depends on the volume of its power supply.
With reference to FIGS. 10 and 11, a conventional power supply includes a transformer 71. The transformer is a central tapped transformer and has a primary winding 711 and a secondary winding 712 for outputting an AC voltage. The primary winding 711 is coupled to a current input terminal 73 through a switch element 72. The secondary winding 712 is coupled to an output filter 75 through an output rectifier. The output filter 75 is for connecting to a load 76.
By increasing the switching frequency of the switch element 72, magnetic elements such as inductors and capacitors applied in the power supply can be reduced in size. However, the high frequency switching will result in the so-called skin effect and proximity effect to the wires of the transformer 71. Power loss occurs in and between the primary winding 711 and the secondary winding 712 due to the skin effect and proximity effect, especially when the transformer 71 outputs a large current.
Further, the output rectifier 74 and the output filter 75 are implemented on a circuit board of the power supply. The AC voltage output from the transformer 71 is transmitted from the secondary winding 712 to the output filter 74 through a relatively long transmission path. An impedance of long transmission path will cause significant energy loss under high frequency or large current situations. The switch element 72 also encounters the problem of switching loss. The reliability and efficiency of the transformer 71 are reduced.
The U.S. patent entitled “Transformer structure”, U.S. Pat. No. 7,705,705 disclosed an improved transformer to overcome the problems mentioned above. The transformer structure has a primary winding and multiple secondary circuit units, with each secondary circuit unit having a circuit board. On the circuit board are mounted a secondary winding and an output rectifying circuit that comprises a rectifying device and a filtering device. An AC voltage output from each secondary winding is rectified and filtered by the respective rectifying device and the filtering device to produce a DC voltage. The DC voltages from all secondary circuit units are then summed up as a DC bus voltage. With the improved transformer structure, the problem of energy loss resulted from the long current transmission path can be solved.
Because the circuit boards are integrated with the transformer, the size of the transformer is very bulky. Electric device equipped with the bulky transformer accordingly has a relative large volume. Furthermore, since the circuit boards are overlapped with each other, heat dissipation for electronic elements such as the rectifying devices and the filtering devices mounted on the overlapped circuit boards is not efficient. The magnetic elements will lose their function due to the high temperature. Thus, the transformer will eventually become ineffective.
To overcome the shortcomings, the present invention provides a transformer with an externally-mounted rectifying circuit board to mitigate or obviate the aforementioned problems.